Thermoluminescence dosimetry (TLD) is a technique for radiation dose measurement. Thermoluminescence is an emission of light which occurs when a thermoluminescent phosphor such as lithium fluoride (LiF) is heated after having been exposed to radiation such as beta, gamma, x-ray or neutron radiation. TLD has several known advantages over other dosimetry techniques such as film dosimetry.
TLD is widely used in the fields of radiation protection, chemical radiation oncology and environmental radiation monitoring. In each of these applications, typically very large numbers of dosimeters must be routinely processed. The time needed to process a batch of dosimeters is dependent on the processing system's degree of automation, speed of analysis or processing of each dosimeter, and computer capability to store information on a large number of dose measurements.
In particular, TLD systems have been developed for monitoring personnel who work in the vicinity of radiation materials, x-ray equipment, etc. In such systems, each person being monitored typically is given a badge to wear that consequently will be exposed to the same dosage of radiation as the person wearing the badge. These badges have consisted of an outer holder which houses a TLD card insert usually containing two, three or four thermoluminescent (TL) elements in crystal form.
Periodically the TLD cards are processed through a TLD card reader to obtain an exposure record for each person being monitored. In the TLD card reader, the TL elements in each card are heated and the thermoluminescence is measured as by a photomultiplier tube and associated electronic analysis circuitry to provide a read-out of the TL integrals and/or glow curve. In automated systems, a batch of TLD cards is loaded into the card reader which then automatically processes the TLD cards preferably with no or minimal human intervention. Some readers also have the capability of digitizing the glow curves and sending the data to an external device such as a digital computer for analysis and extraction from such data of meaningful radiation dose information. A TLD card reader having these and other capabilities is a Harshaw Model 8000 TLD system sold by the Harshaw/Filtrol Partnership, Cleveland, Ohio.
The glow curve is the TL signal intensity as a function of temperature or heating time. A "pure" glow curve is composed of plural overlapping peaks where each peak, in principle, corresponds to one or more TL traps. TLD readers, however, generally provide composite glow curve data composed of the "pure glow curve" plus instrumental and dosimeter background components.
The composite glow curve data heretofore has been analyzed to obtain radiation dose information. For example, LiF thermoluminescence dosimetry is usually based on measurement of the integral of overlapping peaks (4+5) or on the measurement of the height of peak 5. The lower temperature peaks 2 and 3 at about 95.degree. C. and about 135.degree. C., respectively, are generally considered interfering because of their relatively short half-life. Also, the background signal may vary from reading to reading further introducing time dependence in the measured signal, i.e., dependence on time lapse between irradiation and read-out. The contribution of the interfering lower temperature peaks 2 and 3 has been eliminated via a pre-irradiation 80.degree. C., 24 hours anneal or via a 100.degree. C., 10 minutes post-irradiation anneal. In addition, a second reading of the "empty" dosimeter has been made to obtain an estimation of instrumental and dosimeter background which is subtracted from the first reading before measurement of dosimeter absorbed dose.
For automatic TLD readers, both the low temperature anneal treatment and the second reading of the "empty" dosimeter are not practical. In these readers, peak 2 has been eliminated by holding the read-out cycle at a preheat temperature for a predetermined time. The preheat temperature is selected to be sufficiently high rapidly to empty peak 2 but low enough so as not seriously to affect the intensity of peaks (3+4+5) or alternatively peaks (4+5). Other methods occasionally in use involve obtaining the dose information from the area under the glow curve between pre-selected temperature limits. This method has been referred to as the Region-Of-Interest (ROI) method as the temperature limits define the region of the glow curve to be integrated. Background subtraction for automatic TLD readers commonly is done by recalling a previously stored constant background value and applying it to the glow curve data.
Those methods utilize background and low temperature signal subtraction generally are time consuming. The preheat process takes, for example, at least 6 seconds at 150.degree. C. or about 25% of a typical read-out cycle of 24 seconds. Also, it is not possible to empty completely peak 2 without affecting the intensity of peaks (3+4+5) and this results in reduced precision and less sensitivity. The use of the ROI method instead of the preheat method also is problematic. Because of possible variations in thermal contact, the position of the glow curve on the temperature/time scale can change. In that case, the pre-selected region of interest will not necessarily include complete dosimetric peaks. In addition, the high temperature side of peak 2, which is included in the region of interest, can introduce, again, time dependence in the results. A further drawback is that subtraction of a constant background from the glow curve ignores the fact that the background signal is both chip and time dependent and this introduces additional error in the dose measurement.
A computerized analysis technique also has been developed to deconvolute the glow curve into its component glow peaks. This technique is based on the fitting of a theoretical curve composed of the superimposition of three overlapping peaks of first order kinetics, i.e., peaks (3+4+5), plus electronic noise. Each peak is approximated by the expression ##EQU1## where T-Tm.sub.i =.DELTA.T.sub.i, E.sub.i is the activation energy of the i'th peak, Tm.sub.i is the temperature of the glow peak maximum, Im.sub.i is the TL intemnsity at Tm.sub.i, and K is Boltzman's constant. The composite glow curve expression is, therefore, of the form ##EQU2## where c is a constant adjustable background, and the planchet and dosimeter infrared contribution is given by the expression "a exp (T/b)". Y is thus a non-linear function of 12 parameters which can be fitted to a large number of experimental points using a least squares method adapted to non-linear functions with simultaneous variation of all or part of the parameters to obtain the minimum chi-squared. This technique required human intervention by way of selecting initial parameters, i.e., the technique did not enable automatic analysis of unlimited number of glow curves without any human intervention because of the need to select appropriate initial parameters. That is, first estimates had to be made of the peak positions, peak widths and peak heights. Once the composite glow curve has been deconvoluted into its several components, the undesirable separated components may be discarded and only the desirable separated components utilized for measurement of dosimeter absorbed dose.
Dosimetric methods also have been devised for estimating the time interval since an abnormal occurrence of a high exposure. These methods have use in radiation protection applications, accident dosimetry and space experiments. The common approach applied to the temporal estimation of radiation dose, in thermoluminescence and other luminescence related dosimetry methods, is based on taking advantage of the different fading rates of the various glow peaks. One approach measured the "shallow traps" population for various decay times in the ZnS infrared-stimulated luminescence dosimetry and demonstrated the possibility of time read-out dosimetry up to twenty-five days. Sidran, Luminescence Dosimetry with Time Lapse Indication, IN Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., USAEC CONF-680920, (Springfield, VA: NTIS) pp. 883-893 (1969). Another approach used the peak heights fading ratio of the well separated low and high temperature main peaks in single crystal CaF.sub.2 to estimate elapsed times following irradiation up to seven days. Nakajima and Hashizume, On Applicability of TL Fading to Estimation of Time after Irradiation, Health Phys. 16, 782-783 (1969). Another attempt based on Gaussian separation of glow peaks, using the fading peaks ratio in CaSO.sub.4 :Dy, was found to be limited to elapsed time periods up to only eight days. Bacci, Bernabie, d'Angelo and Furetta, Analysis of TLD-900 Glow Curves: Results on Single Peaks Properties, Radiat. Effects 69, 127-133 (1983).
Of particular importance is the provision of accurate time readout dosimetry using LiF:Mg,Ti which is perhaps the most widely accepted material used in TLD. Unfortunately, the use of this phosphor has been hampered by the very complex nature of its glow curve structure wherein the only well separated low temperature peak (peak 2) has a mean lifetime of only about 1.5 days. Accordingly, there exists a need to provide a method and apparatus for evaluating elapsed time between irradiation and read-out in LiF-TLD dosimetry.